1. Field of the Invention
The present invention relates to a diversity receiver, and particularly to a diversity receiver which has an adaptive equalizer and can select a diversity branch having a most preferable transmission quality in an instant.
2. Description of the Background Art
Conventionally, in digital mobile communication between a fixed station and a mobile station or between mobile stations, a diversity receiver which can provide received signals excellent in the transmission quality by removing influence of standing waves generated by fading, that is, the interference caused by differences of transmission routes of signals has been used.
FIG. 1 shows a block diagram of a conventional diversity receiver.
The diversity receiver generally comprises a first diversity branch and a second diversity branch. In more detail, the receivers comprise antennae 200a, 200b for receiving signals respectively transmitted through different transmission routes, signal receiving sections 210a, 210b for respectively converting carrier frequencies of the received signals into intermediate frequencies, received signal intensity measuring sections 220a, 220b for respectively measuring electric field intensities of received signals converted by the signal receiving sections 210a, 210b, a comparing section 230 for comparing the electric field intensities respectively measured by the signal intensity measuring sections 220a, 220b, a switching section 240 for selecting a diversity branch having a electric field intensity greater than that of the other on the basis of the comparison result at the comparing section 230.
Based on the construction mentioned above, operation modes of the comparing section 230 and the switching section 240 are explained with reference to a flowchart shown in FIG. 2 and received electric field intensity shown in FIGS. 3a and 3b.
First, as shown in FIG. 2, each electric field intensity of received signals transmitted in the form of burst in the comparing section 230 are compared with each other at a step ST11. When a received electric field intensity of a first diversity branch is greater than that of a second diversity branch, the first diversity branch is selected at a step ST12. Conversely, when the electric field intensity of the second diversity branch is greater, the second diversity branch is selected at a step ST13.
Namely, as shown in FIG. 3a, by the influence of a standing wave based on interference, when one signal designated by a solid line is received by the first diversity branch, and the other signal having a wave form different from that of the signal and designated by a broken line is received by the second diversity branch, a part of a signal having a field intensity greater than that of the other is always selected in a burst, as shown by solid lines in FIG. 3b.
In such a manner, in the above diversity receiver, a part of a signal having a greater electric field intensity is always selected to improve the transmission quality one by one.
In this case, signals actually received by the receiver generally include signals being the so-called ghost, reflected by obstacles existing around the transmission routes thereof. Then, to remove the ghost, it is thinkable to provide the diversity receiver with an adaptive equalizer in tandem connection.
However, since each signal received by diversity branches has the peculiar transmission route characteristic, the adaptive equalization by the adaptive equalizer becomes meaningless when the diversity branches for receiving signals transmitted in the burst forms are switched to each other in a period of same burst.
In this case, each signal received by diversity branches is transmitted in the form of a packet which is constructed so that a row of code elements in carried by a carrier wave as shown in FIG. 4. On addition, the row of code elements comprises an already-known training signal a whose code elements is known in advance, and an unknown data signal b which follow the signal a and is to be decoded. The packed is transmitted by radio at predetermined time intervals in the form of burst.
A period from a time t1 to a time t2 is called a training mode period, and in this period the training signal a is received in diversity branches. And a period from the time t2 to a time t3 is called a decision feedback mode period. And in this period the data signal b is received in diversity branches.
Therefore, to solve the problem, a diversity receiver equipped with an equalizer as shown in a block diagram in FIG. 5 is proposed (Japanese Patent Application for Disclosure No. 63-51737).
Therefore, to solve the problem, a diversity receiver equipped with an equalizer as shown in a block diagram in FIG. 5 is proposed (Japanese Patent Application for Disclosure No. 63-51737).
The diversity receiver having an equalizer generally comprises a first diversity branch and a second diversity branch, wherein a signal received by a first antenna 300a at the first diversity branch is selected a suitable frequency and amplified by a first signal receiving section 310a, then transmitted to a first demodulating section 320a.
The first demodulating section 320a is provided with an adaptive equalizer so as to remove the ghost contained in the input signals and demodulate them into digital signals. The digital signals demodulated at the first demodulating section 320a are sent to a first training signal detecting section 330a during the training mode period, and digital values of the training signal is detected. Then, each detected digital value is compared with a correct digital value which is informed a first error deciding section 340a in advance to decide a code error rate defined by the total number of different digital values.
On the other hand, in the second diversity branch, a second antenna 300b, a second signal receiving section 310b, a second demodulating section 320b, a second training signal detecting section 330b and a second error deciding section 340b respectively carry out completely the same operation with the operation described in the first diversity branch.
Then, the error rate decided at the first error deciding section 340a and the error rate decided at the second error deciding section 340b are compared with each other at a comparing section 350 so as to select the diversity branch having the received signal of the lower error rate. In accordance with the result of that selection, a switch 360 is controlled to select the diversity branch having the signal of the lower error rate. In other words, only the data signal b following the training signal a received by the selected diversity branch are adopted over one burst interval.
Accordingly, in the diversity receiver equipped with the equalizer, the ghost is removed by the adaptive equalization carried out by the first demodulating section 320a and the second demodulating section 320b, and after the input signals are demodulated by the first demodulating section 320a and the second demodulating section 320b, the error rates are respectively decided by the first error deciding section 340a and the second error deciding section 340b so as to select one of these diversity branches in a burst interval. In the other words, when the adaptive equalization can be utilized the switching operation of diversity branches is not carried out during a burst interval.
The reasons why the error rate after the demodulation at the first demodulating section 320a are different from the error ratio after the demodulation by the demodulating section 320b is that when the received electric field intensity is small, the demodulation accuracy is reduced depending on the degree of the electric field intensity. Particularly, when the received electric field intensity is smaller than a threshold value which is a limit of demodulation at the first signal receiving section 310a or the second signal receiving section 310b, the demodulation at the first signal receiving section 310a or the second signal receiving section 310b can not be ordinarily carried out.
Accordingly, as shown in FIG. 6, even when an average received electric field intensity of the training signal a of the first diversity branch is smaller than that of the second diversity branch, the first diversity branch can carry out a training with accuracy during the training period mode if the received electric field intensity of the training signal a of the second diversity branch is temporarily lower than the threshold value by fading and the like. This means that even if the average received electric field intensity is small in the first diversity branch, the demodulation accuracy thereof is superior to that of the second diversity branch.
Moreover, in such a diversity receiver equipped with an equalizer as mentioned above, the measurement accuracy of the error rate is a very important problem. That is, it is necessary to elongate the measuring time to improve the measurement accuracy of the error rate. However, it also takes a long time to complete the selection of the diversity branch. To the contrary, if the time required for the selection of the diversity branch is reduced, the accuracy of the error rate is lowered.
On addition, since the diversity branch not selected is also operated with the selected diversity branch, the power consumption required for the operation is increased in proportion to the number of the diversity branches.